The present invention generally relates to a flow control valve for a construction machine.
A hydraulic system of mid or heavy construction equipment such as excavators is provided with a control valve having a boom spool so as to control hydraulic fluid in the hydraulic system during the boom-up or boom-down operation.
FIG. 1 schematically shows a circuit diagram of a conventional hydraulic system during the boom-down operation. During the boom-down operation of an excavator, a boom spool is fed with pilot pressure and then moved in a control valve. Here, hydraulic fluid accommodated in a large chamber 110 in a boom cylinder 100 returns to a hydraulic tank 200 through the control valve via a return line 300. That is, the hydraulic fluid that returns from the large chamber of the boom cylinder 100 to the hydraulic tank 200 during the boom-down operation is introduced into the control valve and then flows against the pressure, which is set, to a boom booster poppet for the prevention of cavitation, to the hydraulic tank via a notch formed in the boom spool. The returning of the hydraulic fluid from the large chamber of the boom spool via the boom booster poppet, however, is not so problematic, because, in the case of heavy construction equipment such as an excavator, a stroke of the boom spool is short at an initial section of the boom-down operation so that the amount of the hydraulic fluid returning to the hydraulic tank is not great. However, when the stroke of the boom spool extends beyond the initial operation section, the amount of the hydraulic fluid returning to the hydraulic tank increases so that the returning amount of the hydraulic fluid relatively increases when considering a constant flow passage area. Then, when an allowable amount of the hydraulic fluid returning to the hydraulic tank via the boom booster poppet reaches a limit, the pressure of the large chamber of the boom cylinder consequently increases. This causes a problem of a slowdown in speed of the boom-down operation, and such an increased pressure of the large chamber of the boom cylinder during the boom-down operation adversely affects fuel efficiency.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the related art, and an object of the present invention is to provide a flow control valve for a construction machine, capable of preventing an unnecessary increase in pressure of a large chamber of a boom cylinder during a boom-down operation while increasing boom-down speed.
To achieve the above and other objects, in accordance with an embodiment of the present invention, the present invention provides a flow control valve for a construction machine, the flow control valve controlling hydraulic fluid supplied from a hydraulic pump to a hydraulic actuator, the flow control valve including: a boom valve block connected to the hydraulic pump; a boom spool slidably coupled to the boom valve block and shifted to control hydraulic fluid supplied from the hydraulic pump to a boom cylinder in response to pilot signal pressure supplied to perform a boom-up or boom-down operation; and a boom-down flow control means for returning a part of hydraulic fluid returning from a large chamber of the boom cylinder to a hydraulic tank, through the boom spool only at a predetermined pressure or higher when the boom spool is shifted to perform the boom-down operation, and for returning a part of the hydraulic fluid returning from the large chamber of the boom cylinder, directly to the hydraulic tank through the boom spool and allowing a part of the hydraulic fluid returning from the large chamber of the boom cylinder to join a small chamber of the boom cylinder as a regeneration flow when the boom spool moves over a predetermined section.
Preferably, the boom-down flow control means may include a boom booster poppet mounted on the boom valve block in a path for returning a part of the hydraulic fluid returning from the large chamber of the boom cylinder to the hydraulic tank through a first spool notch of the boom spool to be opened only at the predetermined pressure or higher during the boom-down operation.
Preferably, the boom-down flow control means may include a second spool notch provided to be spaced apart from the first spool notch of the boom spool and to communicate with the path for returning a pan of the hydraulic fluid returning from the large chamber of the boom cylinder to the hydraulic tank so as to return the part of the hydraulic fluid directly to the hydraulic tank without passing through the boom booster poppet when the boom spool moves over the predetermined section during the boom-down operation.
According to the present invention, the flow control valve for a construction machine controls a flow rate returning to the hydraulic tank even when a great quantity of hydraulic fluid returns to the hydraulic tank from the large chamber of the boom cylinder after an initial section of the boom-down operation, thereby providing effects of preventing an abnormal increase in pressure of the large chamber of the boom cylinder and of increasing the boom-down speed, which is also advantageous in increasing fuel efficiency of a construction machine.